Communication using satellite transmission links has become common in recent history. Television, voice traffic, data transfer and a wide variety of other communication types are now routinely sent via satellite. As satellite networks are utilized to provide a medium for more recently developed communications types, satellite network providers and administrators face ongoing challenges in fulfilling this demand. In particular, with the explosion of the Internet as a communications vehicle, the transmission of Internet protocol (IP) traffic via satellite has presented a unique set of challenges. IP, along with electronic mail (e-mail), closed network communications, and the like, reflect the variety of such communication types.
Two of the largest costs for operating a satellite communications network are the use of the satellite's transponder bandwidth, or space segment, and the capital expenditure in both hardware and software associated with the establishment of the network. For many years the use of Single Channel per Carrier (SCPC) satellite links provided an effective communication means for point-to-point connectivity. In SCPC, a satellite channel is reserved for a particular use. Although SCPC provided excellent access, the satellite transponder bandwidth used by a channel resulted in high space segment cost even in low utilization scenarios. In order to reduce space segment costs, there has been a major effort in the satellite communications industry to develop bandwidth management solutions resulting in lower operating costs. This effort has been ongoing over the past several decades. Previously, any reasonable cost could be supported in terrestrial hardware and software, and would be compensated by reduced space segment cost. In today's market however, the falling price of space segment has dictated that terrestrial segment costs must be lowered substantially as well, in order to address a broader market.
One approach to managing satellite bandwidth that has been in use over this time frame is Time Division Multiple Access (TDMA). TDMA divides the satellite bandwidth into time slots that are dedicated to each remote site in a satellite communications network, the network comprising one or more hub sites and multiple remote user sites. The hub sites and the remote sites may be linked via the satellite in a wide variety of architectures well known in the art, including mesh and star architectures, to name two. By time-allocating access to a satellite channel among multiple remote sites, TDMA networks such as the Skystar Advantage® satellite systems provided by Gilat Spacenet of McLean, Va., and the X. Star satellite systems provided by STM Wireless, Inc, of Irvine, Calif. have more efficiently used the space segment and reduced operating costs. In the present terminology, Very Small Aperture Terminal (VSAT) networks typically refer to networks that use some variation of TDMA. Many variations of TDMA have been developed and include Slotted ALOHA, Multiple Frequency TDMA (MF-TDMA) and Selective TDMA (STDMA), to name a few.
Conventional TDMA based networks require an excessively complex and expensive Network Management System (NMS) at the hub. The NMS utilizes centralized network management software to control access to the network by the remote sites, thereby supervising the traffic over the network. The core code for this software is complex and expensive to change resulting in the use of network management logic, which was developed over a decade ago and is not adaptable to changes in network traffic characteristics. Additionally, the capital costs of the hub station electronic equipment are significant. If the network comprises many hundreds of sites, the hub cost becomes a less significant factor as it is amortized over the large number of remote sites. However, for small-to-medium networks of fewer than 1000 sites, the hub capital costs can be prohibitive.
Another disadvantage of TDMA is the limited throughput of the remote sites, particularly during network peak usage periods. TDMA systems utilize equipment that operates in the burst mode to time allocate the space segment. As will be known by one of ordinary skill in the art, because TDMA divides bandwidth into time slots, the aggregate throughput of any satellite channel is limited to the bandwidth of the shared channel, and as the number of remote sites increases, each site's time slot is reduced and the total throughput for each site decreases. As a result, the network average throughput decreases due to the overhead associated with each burst. Some TDMA systems such as MF-TDMA have tried to mitigate this decrease in throughput by jumping (Frequency Hopping) between multiple TDMA channel frequencies. This scheme, however, adds cost and time delay to the system. There are also a number of complex timing considerations with MFTDMA systems that affect network elements interdependently. These factors are, among others, differences in range to the satellite from the earth station, buffer hold time, and frequency uncertainty.
Another commonly used bandwidth management scheme in VSAT networks is Demand Assigned Multiple Access (DAMA). DAMA allows a pool of satellite channels to be shared among multiple user sites on an on-demand basis and is an efficient and logical network topology for satellite systems. DAMA reduces space segment operating costs for networks with remote sites that require full mesh connectivity, but do not require full time communication circuits. Bandwidth-on-demand and rural telephony are examples of popular DAMA applications.
In DAMA, idle satellite transponder bandwidth is available for any remote user site in the network when requested as long as it is not currently in use. The bandwidth becomes available for a requesting user at a remote site upon the completion of its use by another user at a remote site. By sharing the available bandwidth among multiple users, DAMA systems such as the FaraWay™ satellite telephony systems provided by Gilat Spacenet, and the rural telephony systems provided by STM Wireless, Inc., deliver cost-effective use of the satellite space segment.
One disadvantage of DAMA systems is that they are hardware intensive resulting in high capital costs when compared to similarly configured star SCPC networks. In addition to a complex and costly NMS, DAMA systems require one or more network control channels with associated hardware for network management. The control channel manages traffic by passing requests for bandwidth channel use among remote sites and the network manager at the hub site. They require a control channel modem (or modems, depending on the size of the network) at each remote site and at the hub site, and a network management computer and complex network management software at the hub.
Another disadvantage of DAMA is slow communication link set up times. In a DAMA network, when a remote site sends a request to establish a communication link, the request is sent over the control channel to the NMS. The NMS sends an acknowledgement message to the remote and determines bandwidth availability on the satellite transponder. When the determination is made that a channel is available, the NMS allocates bandwidth to the remote by sending a command over the control channel, which tunes a remote traffic modem to the center frequency of the available channel and turns the transmitter on. This process is time consuming due to the satellite round trip times of the remote requests and acknowledgements and the commands from the hub. With several trips up to and back from the satellite, which are a minimum of 240 milliseconds in propagation delay for each direction of the trip, the time to set up a circuit can easily grow to several seconds.
The differences in bandwidth usage by DAMA and TDMA lend each to be more effective in certain applications. Typically, DAMA has been used for rural voice networks and circuit restoral systems, and TDMA has been used for Point of Sale systems, other transactional data networks, and, more recently, IP networks.
With the explosion of the Internet and the rapid adoption of IP as a transport protocol for telecommunications networking, the disadvantages of capital cost intensive VSAT networks with slow response times resulting from long set-up or reduced throughput during high usage scenarios has created a need for a satellite access scheme that that is cost effective and works efficiently in an IP environment. Many companies in the satellite networks business have attempted to adapt TDMA and DAMA for IP transport over satellite, but have had limited success.
A simple application for DAMA in an IP environment is to use it in a manner similar to a dial-up Internet connection. This, however, is an inefficient use of satellite bandwidth resulting in high operating costs. It is akin to dialing into an Internet Service Provider (ISP) via an expensive long distance telephone line and staying connected for periods of time; and, the cost gets very large very quickly. Additionally, IP networks have difficulty functioning properly due to the slow circuit set-up. Routers functioning in a DAMA network start sending route broadcasts to let the destination site know that the route is available. The DAMA system, therefore, establishes a circuit every time the router updates its tables. This may be as frequently as several times a second, but normally is every 2 to 5 seconds. The destination router then acknowledges the receipt of the route updates and recognizes the availability to interchange data.
Subsequent to the adaptation of TDMA and DAMA systems for IP use, a technique was developed for one-way transport of IP data via satellite. This technique uses a standard protocol known as Digital Video Broadcast (DVB) developed for broadcast digital video transport and commonly referred to as an MPEG2 Transport Stream. These systems statistically multiplex an IP stream and encapsulate the IP onto DVB frames using an IP encapsulator. The encapsulator output is fed to a DVB modulator and multicast to multiple receive-only sites via satellite. This technique is commonly referred to as DVB-IP. The DVB-IP equipment manufacturers have adopted a standard within DVB-IP called Multiple Protocol Emulation (MPE). This standard allows any MPE compliant receiver to interoperate with any other manufacturer's IP encapsulator. The receive equipment at the remotes are called Integrated Receiver Decoders (IRD) and are standardized and readily available from a large number of vendors. DVB-IP equipment is produced using large volume manufacturing techniques and shares components with Direct to Home (DTH) television receivers, resulting in very inexpensive equipment.
DVB-IP has proved to be an effective technique for point to multipoint distribution of IP data via satellite. One example is delivering IP backbone connection to far-flung ISPs. Using DVB-IP for delivering downstream data to ISPs is cost effective due to the asymmetrical nature of ISP circuits. The limitation of the DVB-IP system architecture is the upstream (return) path. To date, most networks using DVB-IP are strictly one-way, or use terrestrial links such as dial-up leased lines for the upstream return path. The throughput requirement for the return channel very much depends on the application. If the purpose of the circuit is basic web surfing, the upstream throughput requirement is relatively small. Typically, the downstream to upstream data ratio for ISP backbone access is between 8/1 and 15/1. In many cases, however, the reason for using VSAT systems is the lack of other communications infrastructure. If the station is in a remote area, there may be no dial-up connection for the return path.
An effective solution to the problems associated with the lack of infrastructure is to use SCPC links for the return path. This is what overseas ISP's such as Reach Global Services, Ltd. of Hong Kong use for their Internet backbone connection. An ISP can serve thousands of subscribers with a Committed Bit Rate (CBR) of 2 Mb/s and an SCPC return channel of 128 or 256 Kb/s. Many major satellite operators are currently offering these types of services to ISPs worldwide. Such services include SPOTbytes, the Internet service delivered by PanAmSat Corporation of Wilton, Conn.
SCPC return channels, however, necessitate a separate carrier for each return link. These channels are dedicated full-time to a single link, are usually not fully utilized, and require guard bands between the channels. This spectral inefficiency is very costly, particularly for medium or large networks. Additionally, if the network application is corporate communications, including voice and fax calls, file transfers, e-mail and other services, the demands on the upstream channel are much greater.
Some VSAT equipment vendors have adapted TDMA systems to be used as return channel solutions for DVB-IP systems since IP traffic is very bursty and TDMA lends itself well to burst traffic. Also, the periodic router updates previously mentioned herein function more effectively if they have a constant connection as provided by TDMA rather than the “on request” connection as provided by DAMA. These TDMA based DVB-IP systems, however, suffer from the cost and performance limitations outlined above. That is, the hub equipment is very expensive and throughput is limited when network loads are heavy.
Standardization for TDMA and DAMA based satellite systems has been slow to develop mainly due to the large number of variations required to accommodate the applications and architectures, which they serve. The adaptation of these systems for different applications requires considerable modification. A standard for Internet service over satellite recently promoted by the DVB community is return channel satellite DVB (DVB-RCS) that is based on traditional TDMA technologies. There is also a DVB (DVB-RCT) standard for terrestrial applications and cable systems (DVB-RCC). DVB-RCS is a TDMA based return channel solution designed to serve networks of thousands of sites, and is more targeted at the consumer/SOHO/small enterprise market. This standard was developed in an effort to emulate the success enjoyed by the DVB/MPEG-2 video standard specification. Since the MPEG-2 specification was intended to allow digitally compressed video to be transmitted to a variety of manufacturers' equipment, the MPEG-2 equipment for the hub-to-remote circuit is completely standardized. Many vendors manufacture equipment for this market and it is readily available at very low cost.
The effort to standardize DVB-RCS has foundered, however, due at least in part to the very high complexity and cost of the hub systems. These systems have not performed as effectively as expected. Furthermore, the standard does not address the medium-to-large enterprise network market.
In view of the above discussion, there exists a need for a system and method that provide for the control of multiple remote terminals and their access to satellite communications systems.